You may also be able to use pro-rata, as in a pro-rata employee or a pro-rata appointment.

In England/UK, part time work is anything less than full-time, which is around 35hrs/wk (more or less).
http://www.direct.gov.uk/en/employment/employees/flexiblewor...

I usually rate Hilan quite highly, but if you want UK English then I would be concerned that when doing an Advanced Google Search, it only brings up a handful of hits (57 when I tried)....in addition, it's not an expression I'm familiar with (and I've worked in England almost exclusively since reaching working age).

I would opt for the expression "part-time percentage" as this does have support from UK pages on Google and is a very clear way of expressing the meaning.
See the following:
"Current part-time hours: the number of hours you were contracted to work compared to the equivalent full-time post at the calculation date. This is expressed as a part-time percentage. If you're full time your part-time percntage is 100."
http://www.saul.org.uk/3active/benefit_statements.html

"To calculate your full time equivalent your employer provides us with your part-time percentage. This is the number of hours you are contracted to work compared to the equivalent full-time post expressed as a percentage. For example a member working 20 hours a week, where the full-time post would be 40 hours a week, would have a part-time percentage of 50%."
http://www.saul.org.uk/3active/pt.html

There is also some, albeit less, support for part-time fraction.
http://www.bradford.ac.uk/briefing/we-19-may06.php

I would add the percentage in (), e.g., the employee holds a regular part time position (75%)
I have also seen: "Regular Three-Quarter time Employees" - in the link, but it's from a US site not UK.

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Note added at 5 hrs (2012-06-18 12:46:14 GMT)
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Could also say appointment percentage - as Gad suggested :-)
OR percentage of appointment if it suits your sentence better.