For normal PVM, I'd say 30 is plenty - maybe even 0 is plenty, since you only need to hit most things a few times (takes me 10-11 hits for a greater dragon). Where it really matters is for the bigger, badder things.
Try killing one of the masters in blackthorns dungeon without HLD.
Per UO Guide, HLD does not stack between items, so 30 on glasses and 50 on weapon != 80 HLD, but uses the binomial probability. This answers the question "What is the chance that either my glasses with 30% or my weapon with 50% success chance succeed to penalize the target?" So in this case the chance to apply HLD is [.3*.5 + .3 *.5 + .7*.5] = .15 + .15 + .35 = 65%. With just glasses, of course this is 30%.
HLD reduces the target's DCI for 8 seconds.
During the 8 seconds of lowered defence, you can make a max of 6 swings. Lets assume you're fighting a big baddie - something with 120 weaponskill and 20,000 hp, so you're going to have to hit it a lot. Assuming you have 120 weaponskill, and 45HCI, you'll have a 75% chance to hit. When HLD is in effect, that becomes 85.
Ok, so luck is with you and you land HLD on your first swing. Lets talk about your next 6 swings.
With 30% HLD, the probabilty NOT landing HLD within the next 6 swings is the P(0 hits) + P(some hits, but not apply HLD). P(o hits) = .15^6 = .00001 or 1 in 100,000 so, while I'm sure this has happened to people, it's small enough to discount. P(some hits, but none apply HLD) is a bit more complicated, so lets simply look at the expected value. You get 6 swings, with an 85% chance to hit, so you should expect to land 5 of them. Of those 5, you'd expect that 2 of them apply HLD. What is the probability that 0 of those 5 do? That is .7^5 = ~17% chance that you land 5 hits, but do not apply another HLD.
If you have that 65% chance to apply HLD we talked about earlier, the chance that none of your 5 hits apply HLD is .35^5 = ~5% chance.
Of course this is a bit of a simplification, but the true percentages aren't too far off from there I'm sure.
As a vampire paladin, I simply MUST keep hitting what I'm swinging at to stay alive, and keeping one of these big baddies under HLD is part of that. 1 miss is unlikely to kill me, but 2 in a row is bad, and 3 in a row is terrible.
With a 75% hit chance the chance to miss 2 hits in a row is 12.5%, at 85% it is 2.25% - that difference greatly increases your survavability.
In your case though, I know from other threads that you don't swing at 1.25s, and in the heat of battle against something that hits hard, you're going to spend a fair amount of time < max stamina, which may for a swing or two lower your swing speed even more until you can refresh stam somehow (DF, pot, HSL, normal regen, whatever).
It really depends on what you want to use the weapon for though. If it's a whirlwind weapon, HLD isn't a big deal, same for cases where whatever you're killing only takes a few swings to kill. For a boss killing weapon, the extra HLD is very useful.
(No sarcasm intended, the information is there when I can get my head around the numbers).
