A carefully leveled teaspoonful of pickling lime weighs 2.3 grams. This is very fluffy powder and I imagine that you could get numbers which vary quite a bit from 2.3 depending on how tightly the stuff is packed.
Please be careful with this. It has twice the alkalinity per unit weight of chalk - the more traditional means for controlling mash pH. Don't interpret that to mean that chalk is better. I really like the idea of increasing alkalinity, where necessary, with something that is much more soluble and doesn't contribute bicarbonate.
I don't know how much a "ton" of crystal malt is or what the titratable acidty of the particular malt is nor anything about your water but it is unlikely that the mash pH will be as low as 5.1. If you are basing the supposition that it will be that low on experience then you should be ready with the lime or chalk. If you are basing it on a spreadsheet calculation then be careful. The models that try to tie mash pH to grist component or finished beer color can be subject to large errors when dark malts are involved. For example, if I put my stout recipe and water parameters into one of them it tells me that my mash pH will be 5.0. It usually comes in at about 5.55. This is why I advise people to never add alkali to water or mash unless a valid pH measurement shows it to be necessary. A valid measurement means a pH meter. The strips are not nearly accurate enough. They seem to have a bias of 0.3 pH but it can be more than that and always, per reports I've seen, low. Thus if you use strips and read pH 5.1 it is quite probably that the mash pH is actually closer to 5.4.
If you make a valid pH reading and discover the pH is indeed 5.1 then add the lime in tiny increments. Stir, check again and keep going until you get to the desired pH. You don't want to overshoot.
No, I have never used calcium hydroxide to raise the pH of a mash. I have never, since I figured out what is going on, felt the need to do so. My stout is about 84 SRM and doesn't need it (mash pH 5.55 with my water which has total hardness of about 110 and alkalinity of about 80). My Bock is 26 SRM and comes in at pH 5.33 (with sauermalz) settling in at about 5.40 by the time the protein rest is reached. Those are the only 2 dark beers I do,