Crapshoot,
Try 3.5 grains to start with, just to make sure it cycles. I've been shooting Clays in my 40's for years. Titegroup takes about two tenths of a grain more than standard Clays to achieve the same velocity, depending on your barrel.
I would load 3.2 grains of Clays behind a 180gr Zero bullet to shoot GSSF matches. I started out shooting sub-compact with a 27, it's the only Glock I ever bought. I now own six.... five, complements of GSSF.
That load was pretty wimpy with the 180's, but all it had to do was put holes in paper, knock down 8" plates, and some lightly set pepper poppers. It should be really mild with the 155's.
You should like it!
F1