you need to use
installdir=INSTALL_PATH
fpcdir=FPC_PATH
lazdir=LAZARUS_PATH
also you can share repos/brunch between builds, but you can't mix them up (didn't try it but should work).
to explain it more let say you have:
Build1: lazarus fixes1.6 + fpc 3.0.0
installdir=c:\dev
fpcdir=c:\dev\fpc300
lazdir=c:\dev\laz16
and you want to do second build
Build2: lazarus trunk + fpc 3.0.0
installdir=c:\dev
fpcdir=c:\dev\fpc300 -- > OK
lazdir=c:\dev\laz16 -- > WRONG, because repository already have URL of lazarus fixes1.6 so you need to specify other directory.
so Build 1 & 2 can share fpc repo (same brunch) but can't share lazarus (different brunch)
PS: personally I prefer to not share repo/brunch between builds.