Be careful. On my first cold set, I thought I would have to press really hard using the Sheldon Brown version with the plank. Wow, that was a bit of overkill! I ended up manipulating each side by hand. Use a string line from the head tube back to the outside of the rear dropouts, and use a ruler to measure distance of each length of string away from the seat tube.

The thing I would watch out for after spreading, however, is ensuring the dropouts are readjusted/bent to ensure they are parallel both horizontally and vertically. There is a tool for doing this, but if you are clever enough, you can make up one with long bolts the diameter of the axle, and nuts and washers of suitable size. This also allows you to do the right amount of bending in the correct directions.

Some of the old drop-outs are quite thin steel, which makes that part easier. But newer dropouts can be quite thick, making it more difficult, And if the drive-side drop-out also incorporates the derailleur hanger, that will need to be checked. Another spot to check is chain-ring clearance on the drive-side chainstay; it shouldn't be an issue because it's so close to the BB, but nevertheless, it helps to check.