The reason why you want to use bands and chains is because when you press a straight weight as fast as you possibly can, the weight automaticly starts to decelerate, which contradicts the whole purpose of speed work in the first place.
I train westside so my speed work is basicly this
On squats Week1: 50%, W2: 55%, W3: 60% the percentages are straight weight based, but then on top of this you add roughly 25% of band tention.
And after squats I do block pulls (2 inches from the knees, you can do any deadlift variation you'd like) with the same percentages but I only do 1-10 singles with this weight, and I simply wrap the band around the bar (before i put any weight on) and stand on the band and pull the bands out to the sides until desired amount of band tention.
On bench Week1: 40%, W2: 45% and W3: 50% with eight triples (three closegrips, three medium and two wide, arms give out faster so start close)
I wrap my band beneth the bench and on to the barbell (think of it like the string of a hunting bow). Now days westside uses as light as 30% for 8-10 sets of triples, and they change the bench variation each week, but this method doesn't fit for my gym as It's very minimalistic.
I know that getting exactly 25% of band tention is hella hard because there isn't any real way of knowing where that weight is, and even if you did know it your setup for the bands had to be perfect. But I like to figure it out this way. Test your straight weight maxes on one week, so for me it would be my max box squat bellow parallel, my max flat bench and my max blockpull. Then the next week you test those exact lifts again but now use bands, this will simply show you how much your banded max is compared to your straight weight, so lets say I bench 100 pounds straight weight, and 90 pounds with the bands, then my banded max is obviously 90% of my straight weight max, that means that there is circa 10% more tention at the top. This is the method I have used to figure out my percentages, but remember you HAVE TO use the same band setup each time, and if you want to change the setup, lets say go from a red band to black band, then you'll have to re-test your max, but this time with a black band. If the weight doesn't move fast, then it's not speed work, so always go lighter if you're not sure on if it's too heavy or not.
Hope this helps.