The non-Vac bottles can be quite dangerous in inexperienced hands. If the milk flows when the lamb isn't sucking, you can get milk in the lungs which can cause pneumonia, even death. So you have to practise your technique to get the angles just right.
However you can also cause aspiration with the Pritchard teats if you squeeze the bottle or the teat to put milk into the lamb's mouth, and judge it wrong!
Too much can be much more dangerous than too little. They can recover from too little (within reason) but too much at one feed can overfill the abomasum and cause complications, even death.
A minimum feed is 50ml, if they get 150ml they won't starve before the next. If he's small and very new, lots of very small feeds is much better than a big one.
I would aim to get at least 500ml, up to 700ml, over the first day into a strapping great Texel lamb, some would take a litre on the first day or two but most build up to it.
And a Shetland lamb would take about half what a Texel type would. A minimum feed for a very new Shetland lamb would be 30ml. 100ml or so the target. 250ml might be enough over the first 24 hours, 300ml is plenty.
Anyways, I am understanding you to say that this small lamb is being fed every 4 hours, in which case up to 100ml per feed is probably a good target and you may find he takes less every other feed as he is still a bit full from the one before. (But better to do the little and often while you both get used to it - unless he is resisting the feed, in which case it is often useful to let them build up more of an appetite
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Do you know how to check when they've had enough? And too much?
Hold the lamb up by his forelegs and look down at his exposed belly. There should be a lovely convex shape in both planes; if there isn't, he hasn't had enough.
Then, the way I check that I am not giving them too much is to look at them from above with them standing up. If I can see a distinct "tyre" in front of the back legs after feeding, I gave them too much! I want the belly plump but not bowing out. So next feed, a bit less.
You will soon get the hang of it.