Biology PAG 10.3 OCR

I wonder if anyone can help please ? The biology teacher wants to do PAG 10.3 next week, and wants me to have trialled to find optiumum ratio of yougurt to milk.
The PAG guides the students to set up their own culture, so I'm torn between creating "an optimum" - even if I had time, and that I took it to be the point of their pag.
That aside, does anyone have a ratio which works, please ?
Optimum? Don't they just add an unspecified amount of yogurt to the milk?

I thought the point was to record the pH dropping as the milk is fermented?

I did 250ml of milk and 50ml of yogurt but this was with a BTEC practical. Practical was done on Tuesday 13:00, it was incubated at 46c until the next morning and then put in the fridge.