I just got a bazu stone and am looking at the various seals. Since I already have 40-50% foci for most useful things (healing, fire, cold) as well as spell range etc. one of the items I was looking at was detrimental spell extension (30% at level 70, 27/24/21/18/15 at 71-75).

The question was how EQ treats extension for things with discrete ticks. Does a 5 tick dot become a 6 or 7 tick dot with a 30% boost? Does a 9 tick dot gain 2 or 3? Does a 5 tick dot become a 6 tick dot with 18% (level 74) or does it stay a 5 tick dot? This directly affects whether it's worthwhile doing this, as opposed to spell haste or a 45% boost in magic damage (insect dot, tornado, etc.)

To put things another way, does EQ round up, round down, or something else in cases like this?

It is like with the Direct Damage Foci, they are random. You'll always get at least one tick of extension, if you get a second or third tick added is a roll of the dice. Higher percentage of the base focus increase the probability of that happening, but enchanter parses (excuse me, too lazy to dig up the link) have shown that it is still pretty unlikely to see the second or third tick added.

Hope this helps.

I don't think that foci (or range) is variable. With Vindication of the Dragorn you should see;

Sunscorch/Vengence dots +1 tick
Swarm dots +2ticks
Blistering Sunray/immo dots +3 ticks

On top of those you should already be doing +1 tick of dmg most if not all of the time b/c of phantom ticks.

If there's a chance for an extra phantom tick depending on when you cast I'm not sure. ie: if you take the swarm 9 tick dot, a 30% extention foci should make ie 11.7ticks The question would be if the spell lands at the half way mark of a tick will that make up for the .3 the was holding you back from doing an extra tick of dmg.

yeah u can actually get phantom ticks with focii, happens with shaman HoTs, also happens to dots