great question. my deadlift is lagging behind as well. but I'm lifting as much as I can now. TBH u should try to use 20kg weight plates, so ur total weight will be 20+20+20 = 60kg. Cuz thats the ideal height for the bar to be from the ground.. but also dont add drastically. Lets say last deadlift workout u did 40kg. next day of deadlift workout start warming up with good mornings, stretch your hams, glutes,quads.. Continue the warm up with heavier sets, until u eventually feel its getting a bit hard and let that be the work set of the day. IMO its just my 2 cents and what I would do, but I hold no responsibility whether it is the right way or not.

example
1 x as much stretching as you need.
1 x 5 40kg
1 x 5 50kg
1 x 5 60kg - starting to feel really hard to lift
1 x 4 65kg - failure
(If by any point you feel it is getting really hard to lift,grip failing or whatsoever - thats your workset)

Attempt that weight again the next time u deadlift, should be able to do 1x5, then can continue adding 5kg progressively or 2.5kg progressively

PB: haahha not directly failure, like do until I feel that my explosiveness in jumps are weakening.