I was thrown off by the CBIF image which shows a dark terminal line on the forewing, breaking up near the apex (and my specimen seems to lack a terminal line) but Lynn Scott's image shows a vein pattern that matches mine, and the slightly curving line near mid-wing can be seen in both specimens also.
On top of that, I've seen about 20 light forms of Phigalia titea in the past two weeks (and no light form Biston betularia), so the numbers alone would favor the Halfwing.