Base stats is the "IV 0" value. 40 Base stats for HP, for example means, the lowest HP you can have is 19 on level 5. Therefore, the spread would be 19-20.55 because the value of 31 IV on 5 is 5/100 times 31. I hope this makes some sense?
For example, suppose you knew the exact decimals of your Frockie at something like 20.25, 11.75, 10.1, 12.45, 9.7, 13.4 taking the low base value.
20.25 HP is 19 + 1.25, 1.25 is 25*.05, so your HP IV is 25.
11.75 Attack on 55 base is 10.5(lowest value) + 1.25, ATK IV is 25.
10.1 Defense on 40 base is 9 + 1.1, DEF IV is 22
12.45 SPATK is 61 base (11.1) + 1.35 SPA IV is 27
9.7 SPDF is 43 base (9.3) +.4 SPD IV would be 8
13.4 SPEED is 70 base (12) + 1.4 SPEED IV is 28
You can use the base to find what your IV, statrange is. On 5, it is really easy. Just take the Base, divide by 10, and add 5 to find the lowest value it can be, or the value of IV 0 stats. Just add .05 stat per IV from there.