Loco, I assume it's 15% ofthe Total Income? if so in the taxes cells, rather than just type 15% put this in cell C12
=SUM(C8*0.15) and hit enter
That will calculate 15% of the Total income figure in C8. Then hover your cursor over the bottom right corner of cell C12 and it will change to a little crosshair, click that and drag to the right to over to Cell F12 ad that will copy the formula across to work out the tax for each quarter.
If I've misunderstood and it doesn't need to be the tax of the total income just replace C8 in the formula above with whatever cell you need to calculate the 15% of.
Feel Free to get in touch f that doesn't work or you need any other help mate.