Trying to 'write' a formula in a cell with a variable reference

Hi,

I'm trying (and failing!) to write a formula into a cell using vba which has a variable address;

So far I have tried all sorts of variants on the theme;

Worksheets("subs").Range("u" & insertrows + 13).Formula = "=t" & 12 + insertrows & "/ "l" & 11 + insertrows" (just a snippet of code...)

ie if insertrows = 10, i need to get the following formula into cell U23
=t22 / l21

any help appreciated before I go completely bald!!

Thanks.