I use five magnet slides of different magnet sizes in N-52 and N-42 as just a part of coin testing.
The slide is not foolproof and one needs to be careful about just what can be concluded.
For example a one ounce copper and one ounce silver coin can appear to be no different. Specific gravity would be indicated in a case like that.
And as far as the power of the magnets go, it is possible to have known bad coins test no different than the good ones.
All my slides are two inches wide and made of either twelve half-inch magnets or six one-inch.
Thicknesses I use are 1/8, 1/4, 3/8 and 1 inch.
I also use specimen coins having known compositions as reference. Generally I believe the thinner magnets work better in a number of cases than the thicker ones.
A slide test is just one of several useful items.
It is best used along with other tests such as specfic gravity for silver.
The slide can readily point to a non-genuine coin (like many from China).
But you must be careful about what you have, what you test, and what you conclude.