OK, I see your problem. Over here, the CAA and the airlines accept a ballpark 10 minutes per flight (fast jet and rotary) or 15 minutes per flight (heavies) as a reasonable conversion of miltary to civil hours - that took me some time to add up after 22 years in the military! Thank goodness I didn't then have to convert it to decimal.
However, once you've made the adjustments to calculate your civil hours total, I'd then start a new logbook (or computer program - I use an Excel spreadsheet) from then on. So, say if you have 3250 hours 40 minutes, after adding on the civil frig factor, then you start the new logbook with 3250.7 hours.
Anyway, hope you find an easy way of doing it! I strongly recommend the spreadsheet approach - I found my written logbooks (4 of them!) were 200 or more hours under the real total once I'd transferred everything over!