voltage divider before inverting opamp
Yes, the divider definitely will affect the gain calculation, because its equivalent source resistance (Th&eacute;venin resistance) of 6.23 k&Omega; is in series with one of your 10 k&Omega; gain-setting resistors. The overall gain will be -0.2325, rather than the -0.375 that you're looking for.

If you want an overall gain of -3/8, it would be simpler to forget about the voltage divider and just set up your opamp with 20 k&Omega; input and 7.5 k&Omega; feedback resistors.

Also, be sure to use an opamp whose common-mode input range includes ground.