There was a spreadsheet with estimated ice times from 67-68 all the way up to the years when it was officially tracked. I got it off a yahoo group that no longer exists, but I'm sure there's posters on here who still have it.
Puckerings archive: Estimating Ice Time (14 Mar 2001)
This was one of the earliest methods of estimating it from Iain Fyffes old blog. There are other methods out there. I'm not sure which exact one the spreadsheet used, but they're all fairly similar.
The only flaw to them is that defensive checking forwards tend to be lower than they should, since the formulas are primarily based on TGF/TGA, and a defensive minded forward doing his job of shutting down the opponents would have low totals.