Hi Watso
I use same temperatures as you but don't use a double boiler-just heat milk in an enamelled cast iron pan and if a skin forms just skim it off- still makes perfect yoghurt . I use normal full cream milk- supermarket brand and use a spoonful or 2 of old batch as culture for new. So only costs $1 a litre of yoghurt plus negligible cost of heating milk.
To keep my yoghurt warm while it thickens I wrap the jars in a blanket. put them in a bean bag and then leave for about 8 hours. Keeps it at the perfect temperature at no expense.Perfect yoghurt every time.
A lot lot cheaper than the exorbitant prices for natural yoghurt in supermarket and tastes much better,also much thicker- than supermarket and creamier.
Wouldn't using milk powder would be more expensive than just using milk? Milk powder has a funny taste too .
