I'm using Naturen 280 double strength, calf rennet and I've only had it a couple of months, so it's not out of date, although I see it has no use-by date.
When I first learned to make cheese, I was given a formula to calculate the rennet, based on the quantity of milk. I always use that calculation, even if the recipe says otherwise.
The calculation, based on double strength rennet is .5 mls for each 8 litres of milk. Dilute the rennet with at least 20 times it's volume in cool, boiled water. So based on that formula, I used .7 mls of rennet in 14 mls of water. I use a pipette and small medical measuring cup so I'm able to measure it pretty close.
As I'm starting to see a trend with long floc times, I'm assuming this double strength rennet is not as strong as my earlier batch and I need to increase the dose, slightly??