The same way that you make any other cvar and then use get_pcvar_float() to get the floating-point value from the cvar and assign that to the variable that you want. If you want to use BACKSTAB_ANGLE, you'll need change that to a variable first.
Code:
new Float:fMyAngle
// ...
pCvarPointer = register_cvar() // In plugin_init()
// ...
fMyAngle = get_pcvar_float(pCvarPointer)
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