I have made some progress and made some executive decisions, which can easily be changed:
1) Opted to use the Single FET design, makes more sense to me.
1) BST_PWR (which powers up the boost circuit) has been connected to GPIO1
2) BST_E (which turns on the +24V to the COM terminals) has been connected to GPIO4
Other things I have tried to keep as standard from the original OSPi eg:
Data D5 = 13-GPIO2
Clock D6 = 7-GPIO7
Latch D7 = 15-GPIO3
!OE A1 = 11-GPIO0
So I was trying to avoiding having to include an ADC.
How important is it that the SENSE voltage is available to the software?
Is it used for information purposes or protection to ensure you are not driving too much current?
Can I omit it?