Or maybe another way to phrase it is how to add the % height values of one .TIFF file over another?. Or how to merge two .TIFF files but only a percentage of the value of one and the full value of another.

So essentially, a friend embossed 1/3rd the transparency of the height map of glacial Europe on a map of modern geography, to produce this map below. How can I do the same but with a .TIFF file on QGIS?.

Europe_Iso.png