Seems like you are trying to put too much data in one field. I think that 3 x 0.5 means 3 laps around a half mile track. Invoices manage this all the time where you buy 3 gallons of milk at a set price of x dollars per gallon.
Define these fields: Laps, LapDistance, Total Mileage. Then you can calculation the total mileages as Laps * LapDistance and you can compute a total of the Total Mileage calculation field to get your portal total.
Thank you Phil;
The thought of breaking the "3x 0.5" into two fields crossed my lazy mind at one point. But then I tried hunting for a short cut with the calculation. As with everything else in FM; what seems to be more work ends up being easier in the long run. I'll make the changes.
Thanks again :)
Please note, that you could set up a calculation that could work from that text in a single field, but I think that separate fields makes for a better option.