If you have an incandescent lamp, then warm up should not be an issue. The nice thing about this type of exposure aid is that you are not troubled with any intermitancy effects. You could double check the densities with a densitometer; my Kodak projection print scale is off somewhat. You are developing both for the same time, right? Otherwise, if it is off then you could do some experiments and adjust empirically and make note of the new times.