Add a value every $n

I don't know what to actually call this so if it's already been asked and answered, feel free to point me to it.

I have a Google Sheet with a column of fixed currency values (A), and a column of dynamic values (B) that I would like to increase by x, every n in column A. Ideally the result of this exercise would result in something like (using 1 and 300 as x and n respectively):

 A  B100 1200 1300 2400 2500 2600 3700 3800 3900 4

The part that for some reason I can't get my head around, is that I want to be able to vary the amount (currently 300 in this example) from another cell. So for example in C1 I would input whatever number I want to be the value that triggers the increase, say 300, or 400, or 600, whatever. I also want to have another cell, say D1, that is the number to increase by, so it could be "add 2 every 400" or "add 5 every 800", etc.

I've tried numerous methods and so for every one of them ends up increasing early, or late (rounding issues that I can't seem to solve). How could I accomplish this task? Ideally this would use only existing formulas within Google Sheets.