How do you find accurate slope for these two hall effect sensors?
For the first, yes it means at 1A you will see a typical 185mV. At 2A it will be 370mV and so on.

The second datasheet is not quite as obvious, but if you see the Output Voltage spec, you will notice it has +-4V @Ir. This @Ir means at it's rated current, it will be either +4V, or if the current is negative, -4V.
This is confirmed by the right hand graph, which is for the 50A version. If you look at the 50A point the output is at 4V.   

For your version, which is the 5A version, at 5A you will have a 4V output. So in the same style as the first datasheet, it's 800mV/A.