For a suckler cow, larger than your Shetlands or Dexters would be, we work on half a small square bale of hay per day, a bit more for the first or two month after calving. No cake; maybe a little for a heifer with her first calf for a week or two after calving.
If you take milk on top of what she needs for the calf, I'd feed her cake to get the amount of spare milk you need. The rule of thumb is 1kg cake per 2L milk (over and above what her own calf needs.)
My Jersey rears her own plus bought-in calves, plus gives milk for the house, gets ad lib hay / silage when indoors, and between 2 and 4kg cake a day; I vary the cake according to how much milk I want, how many calves there are, and whether they and the cow seem well-fed. She has one bought-in calf alongside her own Jersey calf, then as each set-on is weaned, another is put on. I usually take a couple of litres for the house, so she's probably being asked for 6-8L above what her own calf needs in the first few months, and then less extra as she'll mostly have just one calf and be giving me my 2L - by then she needs very little cake, just enough to keep her happy while she's being milked.
Hope that helps!