I've given it some thought and I agree that this is probably the better approach. If you (or someone else) has the average save percentage by year going back to 1983, I can post the revised numbers sometime over the next few days.
I'm not convinced that is the best approach. I'm actually pretty sure that using regular season save percentages it is not the best approach for the 21 team league where 16 of those teams made the playoffs.
Here are the average save percentages for those seasons, and the early years of further league expansion:
1984: .873 regular season, .897 playoffs
1985: .875 regular season, .882 playoffs
1986: .874 regular season, .895 playoffs
1987: .880 regular season, .899 playoffs
1988: .880 regular season, .869 playoffs
1989: .879 regular season, .893 playoffs
1990: .881 regular season, .890 playoffs
1991: .886 regular season, .896 playoffs
1992: .888 regular season, .896 playoffs
1993: .885 regular season, .896 playoffs
1994: .895 regular season, .908 playoffs
Overall: .881 regular season, .893 playoffs
I think this is the result of obvious shot quality effects impacting the playoff sample. There was little incentive for most teams to try hard during the regular season. For example, Edmonton Oilers goalies saw their numbers drop late in the year as their teammates went all-out for offence with the division long-since clinched. When the playoffs started they tightened up again, and their goalies' numbers bounced back up. The playoffs were a different game in the 1980s, I'm sure all observers would agree with that, and that's why the numbers increase so much, particularly when defensive teams went deep in the postseason.
Looking at the period from 1995 to 2004, the gap between regular season and playoff drops to .009 (.905 regular season vs. .914 playoffs). This likely reflects the fact that there was more competition for the playoffs, but the league was still fairly unbalanced between the weak teams and expansion franchises and the teams that were well-managed or well-financed. Note that quite a large part of the playoff sample in those years would be great goalies like Roy, Hasek, Brodeur and Belfour, yet the gap is still smaller than it was in the late '80s.
Since 2006, with the salary cap increasing the level of parity and teams having to play hard just to make the playoffs, the difference between regular season and playoff save percentages is just .005 (.909 vs. 914).
The goalies in the playoffs are generally better, but this effect has been pretty consistent all the way along (around .005-.006 per season). Counterbalancing that is the fact that shooters are better in the playoffs as well. The fact that the playoff sample size is heavily impacted by a few teams and goalies is a problem, but I think less of one than the fact that there were such differences in the way teams played in the regular season and postseason. Maybe another option would be to combine the two of them, maybe take an average of the two, to adjust for both of these factors? I'm not sure, just felt it was important to point out the evidence here.