This new procedure is in our ops manuals, approved and all.
Approved by whom? Sounds like something is amiss.
Dan's got it right, except that OCAs are calculated by the relevant country's CAA for
all licensed aerodromes the world over and are based on a careful physical survey of the approach path in order to establish the height above threshold of any existing obstacles. The chart makers - Jepps, Aerad - then throw various prescribed factors into the melting pot, including, but not limited to, aircraft category and the nature of the missed approach procedure, and an DA or MDA is established.
As already stated, a DA includes an allowance for height loss after decision to execute a missed approach, whereas an MDA does not because historically it didn't need to. The chart compilers haven't changed their calculations since these new approaches became common-place, so busting an MDA is still technically against the rules as far as I know and therefore obviously potentially dangerous...
The whole issue of CDFA/CANPA and busting MDAs was apparently debated by a JAR-OPS working party last year but with the arrival of EASA, EU-OPS have now taken up the mantle. Also, as safetypee says, there are differences between TERPS calculations and JAR-OPS calculations of the OCA, but that's a seperate issue.
Hope that helps.
MH152