I am not sure if I understand you correctly or not...but here is my attempt if I do:
In the excell format you can put the holster cost which in this picture is line 1 column D; then line 2 colume D I put 15% for markup and then my total is line 3 column D.
To make line 2 column D automatically do the 15% or whatever percentage you want, just click in the fx box up above and type in the line # and Column number and the percentage you want in this example I typed in: =D1*15% so this box will always do a 15% markup on whatever cost I put in the D1 box.
Then I clicked one time on the D1 box and held down while scrolling down to D3 which highlighted all three boxes:
ANd then in the upper lefthand area of the page you will see Autosum, jsut click on it and now Box D3 will always add the total of D1 and D2.
Then you can change the D1 box from $75 to like $150 and it will automatically change the markup box to the correct 15% and automatically add the 2 lines together in box D3.
Hope this helps, if not then let me know and I will have you email me your document and I will try to make it do what you are wanting and send it back to you.
Ted