Like Desvaro, i started with negatives too. (jump up and hold, slowly lowering myself down)

But instead of lowering down 1 rep 30 sec, what i did was 3-4sets of 8-12 reps 8 secs down, almost everytime i hit the gym (provided my lat doesnt hurt anymore). after about a month, i could do at least 3-4 perfect chin ups

Lat pulldown strength cannot be transfered to a chin up, but as u gain strength doing a chin up, your lat pulldown strength will definately increase

good luck!

Then i think you should just start by hanging on for as long as you can. Build basic grip and arm strength from there. Just try to set a record for as long as u can everytime u hold on to it. Once u can hold on to it for 30-40 sec, then only try the negative chin ups or assisted chin ups with the chair.

persistance is the key