I need enough to keep a high oven running. How big will that need to be?
Not big at all, if you use a Fertilizer (which would require a supply of bone meal and wheat).
Once heated to maximum, a block of charcoal will keep a high oven heated for a long time. We built up a supply of charcoal a while ago and dismantled our farm. Still going strong with a high oven running constantly.
A forestry multi-farm isn't bad either. It's a bit more involved than MFR or T4 golems, but it will give you a use for all that apatite you have sifted.
In my opinion, not worth the effort. Goes along with pretty much everything vanilla Forestry being superceded by better and much more efficient things.
For farms, we have MFR.
Bees and trees, we have Gendustry.
Stained glass, we have many options (MFR, Tinkers', more I think)
Forestry's machines were perfect when mods were simpler, and IC and Buildcraft ruled the tech world. Now they seem nothing more than a relic of a bygone era.