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.

Number → Letter Conversion
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
Often I will grade with letter grades on subparts of an assignment, convert to numbers for averaging (or weighted averaging), and then back into letter grades using the above table. For conversion from letter grades to numerical grades, I use the middle of the numerical range above. Thus, an A is a 95, halfway between 90 and 100. An A- is a 91.25, halfway between 90 and 92.5. Etc. Here is the conversion more precisely:
Letter → Number Conversion
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