If you click on the Help circle while on the Enter Scores screen you will find a description for Entering Averages.
Averages are entered in BridgeScore, usually after the end of session. Typically you enter just a percentage, say 50%, in the NS or EW score column. However you can leave the line empty - see below for more detailed description.
In more detail:
When a board is not played for whatever reason and we do not want to penalise the user in any way, we just leave the traveller line empty. This calculates the scores in such a way that the user is not penalised. I would call this ‘true average’. If you enter 50% for a board that hasn’t been played, then a user that has an overall percentage on the night, say, 60%, is penalised and a user that has an overall score, say, 35% is given a little present.
If you want to give a specific % value – just type 45% or 63% or whatever into the score field on the traveller. NS and EW percentages don’t have to add up to 100%.
BridgeScore doesn’t calculate true A+ or A- (as it perhaps should). It is left up to the user to work those out.
So if somebody should be awarded an A+ (i.e. 60% or higher if their score on the night is greater that 60%) or A- (i.e. 40% or less if their score on the night is less than 40%), you have to look at the result first (without the disputed score), and then enter the appropriate value.