I use percentages to map to letter grades. The percentages mirror the 4.0 scale, except that where a GPA difference of 1.0 corresponds to a full letter grade, I use a percentage difference of 10. The table below shows the conversion from numerical grades to letter grades.
It is my practice not to round the numerical grade before mapping to letter grades by the table. This can be a sore point, so let me explain. For example, I use ≥90.00 as the transition from a B+ to an A. This means that if your numerical grade is 89.9, I map it to a B+ and not an A. It can be heartbreaking to miss a grade boundary by 0.1, I know. But to round up, say, every numerical grade ≥89.50 to 90.00 and map that to an A, means that the transition from B+ to A is actually 89.50, not 90.00. And that would mean that a grade of 89.4 would miss a grade boundary by 0.1. (It would also mean that me announcing the grade boundary of 90.00 is not accurate.) No matter what policiy is followed, some could miss a grade boundary by a hair. Even though there may be some psychological difference between the two situations, I prefer to keep it straightforward by announcing the sharp grade boundary and then following it strictly. I find it helps keeps the process more objective, and does not allow room for subjective grade adjustments, which are almost always unfair.
Number → Letter Conversion 

Numerical Grade 
Letter Grade 
Equivalent on 4.0 scale 
≥ 97.5 
A+ 
4.0 
≥ 92.5 
A 
> 3.7 
≥ 90.0 
A 
> 3.3 
≥ 87.5 
B+ 
> 3.0 
≥ 82.5 
B 
> 2.7 
≥ 80.0 
B 
> 2.3 
≥ 77.5 
C+ 
> 2.0 
≥ 72.5 
C 
> 1.7 
≥ 70.0 
C 
> 1.3 
≥ 67.5 
D+ 
> 1.0 
≥ 62.5 
D 
> 0.7 
≥ 60.0 
D 
> 0.0 
< 60.0 
E 
0.0 
Letter → Number Conversion 

Letter Grade 
Numerical Grade 
A+ 
98.75 
A 
95.00 
A 
91.25 
B+ 
88.75 
B 
85.00 
B 
81.25 
C+ 
78.75 
C 
75.00 
C 
71.25 
D+ 
68.75 
D 
65.00 
D 
61.25 
E 
55.00 