There is no standard in fonts what to use for the value of ascend. Some use the height of, say, the 'k' character. Others enter the highest glyph (including those with diacritics). Basically, that means that using ascend for the first baseline is unreliable if fonts change. The best solution would be to calculate the actual 'max glyph height' of what is actually on the first line so that the text fits exactly to the top of the frame. Using 'cap height' is not a prefect solution too as some fonts have a cap height that is smaller than e.g. the height of that 'k' character.