New data not reflected in fields, bases

Phew! Lets’s see if I can explain this clearly.

Almost daily I go running.
I keep a log of distance and time and get a kick out of recording and graphing the results.
In Airtable, I’m using the chart app to chart several bits of data (DIstance, Time, s per km, s per mile, speed, etc.)

For the straight forward data (duration, time) Airtable has no problem making charts for me, they look great!

But, Airtable won’t let me make a chart with duration. No problem: the current workaround is as follows (example to calculate km/h):

1. Insert Distance as Decimal in “Distance” field
2. Formula -> convert to m (divide by 1000) (new field)
3. Insert Duration as Duration in “Time” field
4. Formula -> convert to s (divide by 60) (new field)
5. In “km/h” field, formula {distance}/{time}*3.6

This then gives speed and so far has been carefully charted every day without fail

But when I try to do something else (e.g. duration for fixed distance - mins per km) the fomula works for all previous entries but never the new ones!

Method:

1. Insert Distance as Decimal in “Distance” field (as above)
2. Formula -> convert to m (divide by 1000) (new field) (as above)
3. Min/km field - Formula ({Time}/{Distance}) (works fine)
   4. Km time - identical field as point 3, but with "number"
   5. Comes up blank, doesn’t chart

I hope that’s clear - what have I missed that the field doesn’t calculate (and therefore won’t appear on the chart?)

I’m hoping to solve this soon as if I need to rebuild the whole thing then it won’t be too big!

Thanks in advance

Edd