Expansion of the skirt is critical to the minie's performance. Go ahead and cast about 15 with wheel weights. Try three five shot groups with 55, 60 and 65 grains FF. Clean between groups and use a fresh target for each group. Then with 15 bullets casted with lead, repeat. The results will tell you what to do.
If the wheel weights don't work for you, save them to cast round balls for smooth bore muskets. Smooth bores are less finicky.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!