Hi, need desperate help! Doing my own accounts and need a formula for the following set up;

So far I have;

Sheet one- monthly summaries showing gross income, allowable expenses, taxable profit for each month.

Sheet two- annual summary so far showing running total of gross income, allowable expenses and taxable profit.

What I need;

A formula or set up that will calculate if I am above the threshold for tax (based on the annual summary maybe?) and then implementing the formula in the monthly summaries that shows the tax, ni and student loan I have to deduct that month, leaving my net wage. Sort of like an average for that month or something. I've tried various ways and it doesn't seem to be working...

Anyone? Sorry if its already been answered- looked through a few pages and couldn't find anything.