1 M3 of solution at 20% ...

1 M3 of solution at 20% strength = 1000L, In that 1000L there is approximately 1000 Kg water, and 200 Kg SHMP.  Your task will be somewhat laborious.  With medium agitation that moves through the tank volume once per every few minutes, assume for now, 200 L/m, you will add approximately 1 Kg flakes or crystals (with the crystals be sure to disperse these widely over the surface of the water, or you will get clumps that take too long to dissolve). You might want a dissolving basket (made of steel, with holes in it) to hold the solid up from any pump suction involved.  Your addition rate may vary plus or minus, depending on initial experience mixing.

If you just dump the material in the tank, add the water, and hope for the best, you will have poor results, and it will take days, not minutes or hours to get the solution you want.

I expect the solution at 20% to have a specific gravity of 1.33, density will be also about 1.33 Kg/L.

Once you know the solution properties, you can easily calculate your mixing ratio as to how many Kg SHMP to add to how much water.  It should be relatively linear in nature, since the volume of mixing for this is somewhat low.