This one is pretty simple: add up the total size of all like weapons that hit, and roll on the Cluster Hits Table column that corresponds to that number. If the total amount goes over 40, split it in half as evenly as possible.
Examples:
Two UAC/10s firing in ultra mode would roll on the 4 table.
Three RAC/5s firing at 3, 4, and 5 shots each would roll on the 12 table.
Two LRM-20s would roll on the 40 table.
Thinking about it:
LRM-15 + LRM-10 (for example, looking at you Yeoman) roll on the 25 table.
MML-7 firing SRMs and SRM-6 roll on the 13 table.
Pros: Ultra Autocannons deserve so much more than the 2 table; on the 4 table the average (7 result) is 3 hits. Fewer dice rolls between weapons, especially units that have multiple cluster weapons of the same type (i.e. multiple SRM-2s). Higher cluster table columns are usually more forgiving.
Cons: A single bad roll can just dumpster all of your cluster hit weapons. The cluster hits table only goes to 40, getting riiiiiiight there or a little bit over makes it more complicated than just doing things normally. Multiple weapons that usually squeak into extra hit clusters (multiple LRM-10s) now get fewer clusters but also greater damage concentration.