Changing part of a formula

In one column I have a time series and another column manually input tender prices, eg 1.2, 1.3 when they are released.
In column C, I have an array formula =($O$2*(Z13-Y$45))+($R$2*(Z13-Y13)).
How can I use excel to pick up the latest tender price which I have entered in cell eg Y13 to replace the 45 un Y$45 in the formula. I have tried IF statement and VLOOKUP but am really stuck.

Thanks in advance.