fix typo, and change phrasing

EIPS/eip-7747.md

@@ -39,7 +39,7 @@ The use of "assert" implies that if the assertion fails, the call frame will con
 
 ### Overview
 
-The execution state of an EVM call frame is modified to include a mapping of `id` (a number 0-255) to "field context".  A field context comprises a modulus and an allocated space of values to perform operations on.
+The execution state of an EVM call frame is modified to include a mapping of `id` (a number 0-256) to "field context".  A field context comprises a modulus and an allocated space of values to perform operations on.
 
 An executing contract uses a new instruction `SETMODX` to set the active field context, allocating a new one in the mapping if it does not already exist for `id`.
 
@@ -172,7 +172,7 @@ When expanding EVM memory, expansion cost will now consider the size of all allo
 
 By restricting that a modulus must be odd, and ensuring that inputs to arithmetic operations will be reduced by the modulus, modular arithmetic operations can be optimized:
 
-* The reduction step for modular addition/subtraction can be implemented using only addition and subtraction.
+* The reduction step for modular addition/subtraction can be implemented without division using only conditional subtraction.
 * Modular multiplication can be implemented using Montgomery multiplication, a family of similar algorithms which have greatly-improved performance compared to the naive method.
 
 ### EOF Dependency