I'd say go overboard with the soap - it's one of the cheaper aspects of shaving, especially as it lasts a fair old while. Try a bit more than that next time and see where you go. I'd say as well, add water gradually. I've had foamy, dissipating lather from not enough water. It can be a tricky game trying to get it right. There's also little harm trying out a soap without shaving, just seeing how much water it can actually take and transferring that to your shaves down the line. The heavily-tallowed UK soaps such as OSP and P&B take on a
lot of water. Like, loads. If I'm face lathering I tend to gather some damp, soapy paste on the brush while loading, transfer that to my face and add water gradually until I hit the sweet spot. Usually I end up with too much lather but, like I said, it's cheap enough and I'm not going to lose sleep over it!
Hope this helps and keep up the good work!
OSP is great when you get it bang on!