Modbus Addressing in a Data Type Structure

Hello, I’d like to set up a Data Type, reference an address as a parameter (FirstAddress) type Integer and then reference that address with an offset to each tag in the structure.
For example if my Data Type was all Boolean I’d put in the first address 25 and the OPC Item Path would be [OPCDevice]C{FirstAddress}, this works fine…
Now if this where say 10 consecutive Boolean addresses the next address would be something like [OPCDevice]C{FirstAddress}+1.
If I put that in I get [OPCDevice]C25+1.
Adding parentheses ({FirstAddress}+1) gives [OPCDevice]C(25+1).
I even tried creating expression tags for the address, say AD1 is {FirstAddress}+1 so it shows as 26, all good. But when I put it in the OPCItem Path as [OPCDevice]C{AD1} it tries to read that exactly and doesn’t put in the value of AD1??
I’ve done similar references in the OPC Item Name with memory.
Any Ideas??