HELP: Setting INSTALLDIR with CA breaks costing

[Alistair Mackay]

All,

Trying to set the installation directory from a path stored in the
registry. I can get the value form the registry into a property, but
find that if I try to use a CA 35 to set INSTALLDIR to that property,
it causes an error if inserted in the sequence anywhere before the
completion of costing (MSI debugger shows INSTALLDIR undefined until
costing complete), and totally screws up the cost calculation if
placed after costing (total space required is subtracted from C:, but
each feature cost is 0KB, even when the target is not C:).
How can I make it re-cost based on the new value of INSTALLDIR (a path
on a different drive).

TIA.