I'm not an "Expression expert", but you might try nesting IF statements.
if (Chortime() >.5, if( Chortime()<1,Chortime()*1000{,0}))
Just a thought
Just tried it: If(ChorTime()>0.5,If(ChorTime()<1,ChorTime()*1000,0),0)
It works
You can leave the ",0"'s off as they seem to be optional and zero is the default.