You could try making a mark on the body of the guitar with a pencil. Or you could change the strings one at a time.

First of all, it is not a good idea to take all six strings off at once. Even on a guitar with a fixed bridge, over time as the parts settle the neck and neck joint becomes accustomed to string tension. Releasing it entirely and pulling it back can cause problems. Except when necessary for repairs, it's always best to replace one string at a time. Do that and you’ll never have to reseat the bridge. When you read the process of putting the bridge back you’ll understand why. I have used the process below for acoustic and electric archtop guitars (the kind with floating bridges), mandolins, and banjos. Does your bridge have individually adjustable saddles? Most archtop acoustics do not but I have seen a few that do. If so, have you moved them since taking the strings off? If you have, adjust them to about middle position before starting. If not leave them where they are. Lay the guitar on a padded surface on its back. Do not pick it up or turn it on its side until you have finished bringing all strings back to pitch. If you turn it while the strings are loose the bridge can slide off and you will have to start over. Put the saddle in approximately the right place and install all six strings but leave them loose. Tighten them just enough so there is no visible sagging between the nut and bridge but nowhere near tight enough to play. The strings should be slack enough so that the bridge moves easily underneath them. Starting with the low E (6th) string, use a tape measure to measure the distance between the middle of the nut and the middle of the E-string saddle. Divide the distance by two to get exactly half. Half the distance between the middle of the nut and the saddle should be directly over the 12th fret. Adjust the low-E side of the bridge until the mid-point of the string is directly over the 12th fret. Don’t worry about getting the mid-point exactly over the 12th fret. Unless you are using a straight edge with precise gradations you won’t. The idea is to get the bridge close to the right position and to use the string’s intonation to get it exactly in place. Do the same thing with the high E (1st) string taking care not to move the low E side. Putting a spot of low tack tape next to the low E side of the bridge helps. (DO NOT use masking tape! It can mar or pull off the finish. If you don’t have low tack tape for woodworking use blue painter’s tape and don’t press hard.) Now bring the low E and high E strings up to pitch but leave the middle strings slack. As each string comes to pitch give it a gentle tug away from the fretboard to stretch it out so it stays in pitch. Otherwise the natural stretch in the new strings will give you false results. Once both E strings are up to pitch, remove any tape you used. Once both strings are up to pitch, first set the intonation on the low E string. You can do this by ear but will get better results with an accurate tuner, perferably one with a needle. Tune the string so that the harmonic at the 12th fret is on pitch. Then fret the string at the 12th fret. If the note is higher than the harmonic, carefully move the low E side of the bridge a tiny bit further AWAY from the nut. If the note is too low, move the low E side of the bridge CLOSER to the nut. The operative words are “a tiny bit.” Do this slowly and be careful to slide the bridge. Be careful not to tilt the bridge - the feet must remain flat on the top. Any time you move the bridge, retune the string so the harmonic is exactly on pitch and repeat the process. Once you get the low E string harmonic and 12th fret note the same, mark the position of the low E side foot with tape. The repeat the same process on the high E side. Once you get the high E site to intonate correctly, recheck the intonation on the low E side. Now is the time to make adjustments because moving the bridge will be difficult when all six strings are at pitch. Once both the low and high E strings intonate correctly, mark the position of the bridge with low tack tape. Then bring the remaining four strings up to pitch. Best to do this gradually so as to not shift the bridge. Tune each string low-to-high up to about ¼ pitch, then to ½ pitch, then to ¾ pitch, then to pitch. Recheck the intonation on your low and high E strings. If they are still correct and your bridge does not have adjustable saddles, remove the tape and you are finished. The intonation on the A-D-G-B strings will probably not be 100% correct though it may be close if your guitar has a compensated saddle. Don’t worry about not getting all six strings right on. They should be close and tuning a guitar is a compromise anyway. Pythagoras goofed. Pressing a string down slightly changes its length and he didn’t take that into account when dividing the octave. Hence the expression, "close enough for jazz." If your guitar’s saddle has individually adjustable saddles, now is the time to set the intonation for each string. If the fretted note is higher than the octave, move the saddle away from the nut. If the fretted note is lower move it closer. And don’t drive yourself crazy if you can’t get it exactly perfect. As long as it's close your guitar will play in tune. Now do you see why you should never take off all six strings at once? Hope this helps! JMAC