If the OP is not doing res changes, towards the end of bloom, ppms don't mean diddly, as plants do NOT use all components of the nute system exactly at the ratios at which they are provided.

The ratios in the bottle are an 'idealized' ratio for cannabis the genus; differences among species and of course strains will cause unused components to build up, giving a level of 'junk' ppms, that the plant just doesn't need and can't use, when the res has gone too long without a changeout to start back at the idealized ratios.

If you were to have a sample of your res solution tested at a lab for individual components, you would find a NON-ideal ratio among them, with a lot of the conductivity being added by components in excess, leaving less 'space' left over for stuff your plant actually needs at this stage in its life.

The moral of the story is, change out your res for a nice freshie as often as it is practical. Otherwise, yield and plant vigor suffer.

Epsom salts, by the way, are NOT a substitute for CalMag. Epsom salts are MgSO4- that's IT. There is no other necessary mineral content in there- the Calcium is missing- and plants do not use enough Mg to justify adding the level of ES to bring the base ppms to the same level you'd use CalMag for, as that is a broader nutrient, with minerals, not just a metal salt.