If you open a premium box, chance of L = 20%, chance of E = 80%. (my sample size is a couple thousand premium packs, which is x5 premium box)
Depends how far off you are from a meta build - if pretty close, maybe try your luck with premium packs (don't do boxes - they give you less for tokens); but if far away, I'd recommend saving tokens on portals and spending it to refill fuel for L drop chance from portal boxes. But if you have enough tokens/money to spare, but premium packs away. Note though that most good L-Ms have a drop rate of about 0.1%