I interpolate between 2 values of GWC that I "know" from some rough tests:
a/ when 10% behind in EPC (and not on roll)
b/ when 4 pips ahead in EPC (and not on roll)
For rolls vs rolls I've tested that the /a/ value is about 16%, whereas the /b/ value is about 50%. Around this range, the relation between EPC difference and GWC seems to be close to linear, so I just count the value of a pip and adjust GWC based on that.
My tests were really rough - not more than 200 positions overall - so please take it with a grain of salt. This method is certainly far from being something I'd like to share widely. It's really young, and I've just "invented" it for myself as an improvement over iSight. I needed a method of evaluating GWC as I am a match player, and iSight was too often 3, 4, 5% off. Even though it's quite young, it has served me well already. Of course the big downside is that one has to be really good at estimating EPCs. I'm still far from that, but the method gave me some good results already.
A quick example:
                     |
| ||||
| XGID=-CDCBA-------------bcba---:0:0:-1:00:0:0:3:0:10 | |||||
| White on roll, cube action? | |||||
eXtreme Gammon Version: 2.10
White has "pips" and Blue has "rolls" position.
White is a bit off from the ideal minimum-wastage position, but he has only 8 checkers left. That usually results in about 7 wastage, so use 45 EPC. Actual value is 44.7.
Blue is 7n+1 = 50 EPC, plus 1 for the OK distribution (with several checkers off and odd number left, he is usually in the +1 to +2 range, depending on the distribution). Actual value is the same - 51.
I remember from my tests that in pips vs rolls, the 10% EPC trailer has about 20% GWC if leader is on roll.
So here if Blue had 45 + 4.5 = 49.5, that would be about 20% GWC. If he were 4 pips ahead (41 EPC), he would be about 50% GWC. Distance in GWC is 30% and in EPCs: 8.5. 30%/8.5 is about 3.5% per pip. Blue is 1.5 pip behind the 49.5 EPC mark (20% GWC), so he is worse by 1.5 x 3.5 = about 5% GWC. This gives 15% GWC. Actual value is 14.9%.
On a side note, the iSight gives Blue 22.7%.