How to adjust stewing time for different weights of meat?

Simple scenario: I've got a Madras recipe that tells me to stew 2.0 kg of Lamb shoulder meat in the oven, covered with foil for 2,5 hours at 180 deg Celsius, then another 30 mins at 200 deg without the foil.

Now I'm cooking for only a few people, so I have 0.6 kg of meat, i.e. only 30% of the recipe's weight.

So, my question: How should I adjust the cooking times? Any useful rule of thumb?

My initial thought was be to factor times down to 0.6 / 20, i.e. respectively 45 and 10 minutes. Then again, this seems on the low side for a tender stew (or is it?). This is probably because of those 2,5 hours, maybe 50% would be to ensure the meat's cooked, and the other 50% is to ensure it's tender; and those halves are to be adjusted differently?

Suggested questions "that may already have your answer" has one great title , but question nor answer helps me answer my question. Searching seems to yield zero results, though my queries may've been wrong.