Noob when it comes to expressions

[figge]

Hi all,

Question for the ones experienced and well versed in ArchiCAD. I'm trying to get two measured zone areas divided as an expression, generating an FSR for our project. Both zones have their own expression for the measured area.

I've got the site area zone expressed individually as

STRCALCUNIT ( {Property:Zone/Measured Area} )

...and the GFA Zone expressed individually as

STRCALCUNIT ( {Property:Zone/Measured Area} )

In my mind the FSR expression would be 

( STRTONUM ( {Property:General Ratings/GFA} ) * 1 m2 ) / ( STRTONUM ( {Property:General Ratings/Site Area} ) * 1 m2 )

I get no errors with the expression but still no value...?? What am I doing wrong / missing..?

As I noted, I've got a decade and half experience in AC but only very recently got my head stuck into expressions and feel like a complete beginner again.

Operating system used: Windows 10 Pro