Pedro's sells a specific tool to do this job of holding the BB cartridge tool snugly against the splines. The threading on the BB bolts is a finer thread than most hardware stores generally stock in their Metric bolts bins. The Pedro's version has a knurled collar that can be threaded down to meet the top of the tool. You won't need a stack of washers and you can back off the tension as the BB cartridge begins to turn and back out. Most of the time, you wouldn't need this but if the BB is rusty or has some tenacious thread sealant, then this feature is useful.

http://pedros.com/products/tools/ped...socket-holder/