That's a good question. I'm not sure what the best way to handle it is either. Maybe allow one extra tractatus per 10 years if your score doesn't go up sufficiently to allow one by the normal rules? I wouldn't want to overcomplicate things.
2 solutions:
a) You can only write 1 tractatus each of level {5 10 15 ...}. If you wait until your Art gets to 16, your first tractatus may be level 20 but your second will be 15 or less unless you advance your Art to 21.
b) You can revisit a previous subject and rewrite the same tractatus at a higher level. Since it is the same, you cannot gain the experience twice.
The first solution seems easier to track, reduces tractatus in circulation and has no penalty for early production. Moreover, Spring covenant may start with mostly low tractatus and very few summae if the build point costs are adjusted.