I can't answer your question directly but this might work: Try swapping out the Fritz engine for Stockfish, its engine parameters have a playing strength setting, you can set it 0-20 and it will always play at that. Not the same as choosing a rating, but might help.
Has the advantage that on full strength it plays a lot better than Fritz, especially because it will use as many cores as you give it instead of the single core Fritx limitation.
@nigelnorris thanks for the suggestion. If I understand correct I'll have to run stockfish with a client like Arena and then I can configure the strength level. I'll take a look into that, thanks a lot for the info.
FYI Fritz 15 let's you pick multiple cores, my machine has 8 cores and it defaults to 4 but you can set it to 8.
Oh my mistake, I thought Fritz to be the single core version and Deep Fritz to be the multicore version, didn't realise that they did away with that at v15, I only have v13. Either way I don't know what Arena is, I just downloaded the Stockfish engine from its home site then used the engine management feature in Fritz to install it and then set it as the main engine. Once it's set up Fritz is indistinguishable from before except that at any time you can use engine management feature again to change the playing strength.
In Fritz 15 you can play against the engine, and set the strenght based on "rating" for it to play at. From what I can tell there are two ways to do this.
In both of these you begin the game in the starting position.
What I've been doing is playing in Friend mode and the rating increases as I win and decreases as I lose. Many times I play a game, reach a winning position, and then lose in the endgame.
What I'd like to be able to do is to start a new game with the winning position and the have Fritz play at the same "rating" that I previously lost to.
I couldn't figure out how to setup a position and then have it play from there limiting the strength with a "rating". You can specify the time/depth but not the rating from what I've discovered.