Typically, A criticals get an instant death result on the top entry (100). B gets instant death on the top two (96-99, 100), C gets instant death on the top three (91-100), D on the top four (86-100), and E on the top five (81-100).
As a simplification, you could have one table with results from 1-120, and then give a bonus on the roll of +5 for B, +10 for C, +15 for D, and +20 for E. You'll get more repeats than having separate tables, although you could deal with that to some extent by having more results per table (for example, currently 21-35 is all one result, and 36-45 is one result; those could be split up).
You could stretch the table out more and give bigger bonuses, but the differences at the low end of the tables aren't that significant, and doing so would magnify them. E.g. it's possible to get an E critical that only does a couple of points of damage, but if you gave a larger bonus that probably wouldn't hold up.