Yah, when I did this I took lifts I had actually recently performed which felt pretty close to my 5RMs, then used that exrx.net estimator to estimate my 1RMs.
I made my own spreadsheet and I did all everything in kgs, but when it came to adding 5lbs to the upper body lifts and 10lbs to the lower body lifts, I just added 2.5kg and 5kg. Plus... I used the excel "ceiling" function...
I quickly skimmed the calcs when I was done and it mostly looked like my heaviest sets in week 3 of each iteration were either 2.5kg or 5kg above the previous month, so I figured it was close enough.
but yeah, it's probably a combo of all that making the numbers a little wack. I'm gunna say it doesn't matter much inn the long run