Well it's turning out rather complicated. It's a little bit better now but not optimal. I'm gonna try something new for WHS but that will take a bit of time.
[Edit] 1 hour later, i tried a few things but the part that draws a horoscope is quite complex and i'd have to redesign things to improve this situation. Frankly i'm not too happy with this but for now it's the best i can do. You can see the problem illustrated when you move a horoscope in time per minute using the arrows. The housecusps dance a bit round the signcusps, sometimes exact, sometimes one pixel off.
[Edit 2] If you still see cusps that are off too much, please redownload.