If you search on here, I have written a guide on how to turn morphine ER tablets into liquid morphine solution for rectal administration (I get 18+ hours of pain relief out of a few hundred milligrams of rectal morphine, when it would take like... 500-750mg orally for me to produce the same pain killing effects.
As for how to gauge your dose, depends on your tolerance but here's the thing. Morphine isn't like hydrocodone or buprenorphine at all, its WAYYY stronger especially compared to the doses of hydrcodone and buprenorphine you were taking, it's a whole different ballgame. Morphine is essentially heroin. Heroin turns into morphine into the body upon admninistration as heroin itself is inactive, it just gets the morphine across the BBB much faster
I have never heard of 40mg morphine tablets being made, what brand are they / where are you from, generally? like, USA? Europe? I'm pretty well versed with all the morphines as it has been my DOC for about ten years but I pretty much only mess with the [M]200mg, [M]100mg ER, mallinckrodt ER tabs and the 15 and 30mg IR tabs or Roxanol.