1250 fps with a 125 grain bullet from a shorter barrel is NOT what I would consider a "mid range" load. From a short barrel like that on the 640 it will feature a rather extreme level of muzzle flash and blast. In addition recoil will be a bit uncomfortable.

The Cowboy loads suggest are actually Minimum Magnum and would be gentle shooting. Unfortunately the bullet used isn't ideal for SD purposes.

The suggestion I find the most suitable is the Speer 135 grain 38 +P Gold Dot. Hard to find and likely a dollar per round but it's a combination that was specifically engineered to perform well out of compact revolvers such as the 640.

Personally I use this bullet in a sub Magnum hand load and it's very easy shooting out of my 2 1/2 inch model 19 with fantastic accuracy. BTW, consider this a hint to all revolver shooters, start hand loading. When you do you'll find that you can hand load 38 special for about 12 cents per round with plated bullets and 357 Magnum using premium bullets for under 30 cents per round.