Help with calculations

I am trying to figure out the financial affect of taking some unpaid leave this summer.
How do I do it?
If I take say 6 weeks unpaid leave. How does it impact on the tax bill the following month? Do I just add the allowances unused and reduce the tax bill accordingly?
Marginal rate is 42%

Perhaps you could use the "end of year tax returns" view of [broken link removed] to compare the "normal" and "unpaid leave" scenarios if that would help?