Don't know the exact angle needed, but do have a few thoughts for you. Have you used this spokeshave before? The first time I picked up a spokeshave, it was very frustrating for the first hour or so.

There are many factors that work together to make a spokeshave work well, or not so well. The blade depth, the angle of attack on the work, and the grain direction all play a huge role. The grain direction is probably the most important. Just like with a plane, you want to go downhill or with the grain. That can be difficult with many strips next to one another and some grain going in differnet directions some times. Also I found that rocking the spokeshave forward or back, so the wood was pressing against the leading edge or trailing edge of the sole could help in certain situations. The trailing edge is larger, so I imagine that the manufacturer expects you to put most pressure on this edge. I found though that my spokeshave cut cleaner when I put the pressure on the leading edge and let the blade ease into the wood. You can skew the anlge of the shave to the direction you are shaving, and that will effectively lower the cutting angle of the blade against the wood.

It's hard to write about this, and easier to just get a feel by doing it. I'm not so sure blade angle for a spokeshave is so critical. I may be wrong, but I think it doesn't matter as long as you are in the ballpark, which you are. Try the shave out on some scrap wood you have lying around. Adjust the blade depth and try attacking at different angles to the wood. ALso pay attention to the grain. Once it works for you on some scrap, you shuld have an easier time on the boat. One last piece of advice, I sometimes put one side of the blade deeper than the other. Then when I take the tool to the wood, I can put the point of contact at the agressive side or the fine side without stopping to fuss with the tool.

Of course, if you've had this shave for a while and can use it fine normally, just ignore my post. You will probably get the angle number from someone.
Hope this helps.
-Dan