I've never used Fomadon Excel but, in common with other film developers, it can probably be reused a number of times if an increase in development time is given (usually around 10% for each subsequent film). This compensates for loss of developer activity. As an example, if your initial development time for a given film is 6 minutes, the next film would receive 10% extra development, i.e. 6 1/2 minutes (rounding down slightly). The next film after this would receive 10% of 6 1/2 minutes and so on, until the theoretical maximum capacity is reached. I say theoretical because I suspect the developer would have yielded its best sometime before.
An easier and (IMO) more consistent method of development is to use use it as a one-shot solution. Using it 1:1 would mean diluting your developer with an equal amount of water. The benefits are greater consistency, sharper negatives and better rendition of contrasty subjects. Regards, Taxor