Also, if you don't have a pH meter, just wait 16-24 hours. For meso culture that doesn't produce CO2 (no diacetylactis or leuconostoc), culture the mother at 86F. For meso culture that does produce CO2, culture at 72-74F. For thermo, it depends on if it has S thermophilus or not. If it does, culture at 104F. If it doesn't, say if it's a pure LH, you can go higher, to 110F. Use a 4-5% bulk equivalent of DVI culture as the seed for the mother. Practically, this is something like 1/16 tsp per gallon of milk. Meaning if you're trying to make enough mother culture for a typical home batch of 4-5 gallons, you don't need to make very much and you use very little DVI powder, maybe a pint of mother starter and just a pinch of culture, and you usually don't even need to use all of that pint for 4-5 gallons..
Also, use nonfat or skim milk and boil first to sterilize. It's like making yogurt, like Francois said. If you want to be really scientific, add enough dry milk to get to 10-11% protein. This will increase the buffers and let you almost double the amount of viable culture. But if you do this, be consistent with all your starter amounts and media.
IMHO, this produces a superior cheese with less hassle. My pH and time schedules are better when I don't wait for DVI to wake up. Also, it's easier to over or underdose with DVI culture when making small batches, so there's more inconsistency. Meaning with a mother, you can be more exact in your initial bacterial inoculation amounts, and this helps with consistency.