Assuming you don't have any LTA, glider, helo, etc. time... Single engine + Multi should equal TT. Also, day + night should equal TT (if you have a day column in your logbook). FTD or PCATD or SIM time does not include TT. TT should only include TOTAL FLIGHT TIME.

Find out where the error is and put a note on the current page. There are many variations:
1) Corrections for calculation error on ##/##/#### or
2) Correction for calculation error on single engine column to reflect additional 1.2 hours logged on ##/##/#### or
3) Correction for calculation error on total time column to reflect additional 1.2 hours from page 22 of logbook 2 (if you have multiple logbooks) logged on ##/##/####.

Everyone makes calculation errors on logbooks, I've done it too and I'm on logbook # 3. When I total up the page on my logbook I now always verify that Single + Multi AND day + night = Total time on every page. This way I don't have to deal with the aggravation of trying to find an error on the last 10+ pages.

If you can't find the error or if you have multiple logbooks, I suggest that you REDUCE the flight time instead of increasing it. For example, if you have 1000 SE and 200 ME but your total time is 1202... AND YOU CAN'T FIND THE ERROR, then decrease the total time to 1200.

I'm tempted to buy an electronic logbook... it's probably something that I'll look into when I start flying 121. Good luck with the CFI job.