We evaluated your issue a bit. Well, your issue is due to the fact that in the given template file, the saved formula values are incorrect for row 31. If you do not calculate formulas in the workbook, we just use that value (read) from template file for rendering. When opening the file in MS Excel and re-save it, MS Excel will calculate formulas automatically and save the correct value into the re-saved file, so there is no issue when using the re-saved file to generate the image. To simulate MS Excel’s behavior, you should calculate formulas before rendering, see the lines of code for reference:
e.g Sample code:

Another concern is we don’t want to calculate all the time.

Is there any way to know how many formula or how much the complexity of the formula inside the workbook so we can make better decision whether to calculate or not?

I am afraid it is not possible, we cannot know the complexity of the formulas until we calculate it actually. And, even we calculate the formulas, it is hard for us to describe the complexity involved. You can only get the total cells (count) which contains formulas. You can gather the count by checking Cell.IsFormula attribute, but we don’t think it is much useful for you to determine whether the calculation is simple or eating high resources(time, cpu, etc.) .