A magazine I saw today featured a JIG for cutting Post Formed Countertops
such as the one a reader had a problem with last week (and posted here).

Basically, it is two pieces of 3/4" MDF or plywood (one bout six inches
longer than the counter top is deep and the other about six inches longer
than the backsplash is high/tall.

These are joined at right angles so as to lay on the counter top (with the
counter top positioned "right side" down).

A strip of wood is fixed to the edge(s) to guide a "skill saw" base plate
as it cut through the MDF Support and the Counter-top below, then run "down"
the back side to cut the back splash in the same fashion.

With the "good side" down, it appears to meet all the requirements for a
clean cut and could be adapted ( I suspect) to cutting the 90's our OP
required.

Sorry I forgot to note the name of the publication!