[AccessD] A2003:Calculating Percentages - and their currency values

Hello all
In Australia we have a sales tax apply to all purchases. It's called GST
(Like VAT in Britain)

At the moment the Govt. has set this Tax at 10 percent. Makes the maths very
simple by 
Just adding 10 percent to each currency amount.

To calculate any TAX inclusive amounts we just divide the grand total by
1/11 (one eleventh)

So far so good.

Until the Govt. changes the GST Rate (they said they won't but we all know
they eventually will)

So I don't want to hard code "Divide Grand total by one eleventh" to get any
GST inclusive amounts.
If the GST amount is changes to say... 12.5 per cent - it won't work -
obviously

So.for all the mathematically inclined folk out there..
Is there a clever formula for working out GST Inclusive prices regardless of
the Current TAX rate?
Just like the one eleventh business works for 10 percent?

Many thanks

Darren