It comes as part of a deal outlining the future of teacher pay and conditions which also links all such pay decisions to performance management.
Other recommendations from the rewards and incentives group cover salaries for experienced teachers who want to stay in the classroom, under the "excellent teachers" scheme, pay bonuses for science and maths teachers, and the future of special educational needs allowances.
The Assembly government is responsible for performance management in Wales, but not pay and conditions.
The recommendations have been contested by the National Union of Teachers and the National Association of Head Teachers, which are not members of RIG, an organisation made up of the Westminster government and the education unions which have joined it in "social partnership".
But the breadth of the interests represented by RIG means the plans are likely to be accepted by the School Teachers' Review Body when it reports in December.
A 2.5 per cent pay rise for all teachers was recommended by the review body and accepted by the Government last year to cover 2006-08.
RIG-commissioned research on how part-time teachers are paid uncovered huge variations in how salaries were calculated. A total of nine different methods were identified among the 78 English and Welsh authorities that responded.
They showed that some teachers were missing out because no account was taken of the fact that the morning sessions they worked were longer than afternoon ones.
Other authorities did not pay part-time teachers for the work they did outside school hours.
A new standardised method being recommended by RIG will pay teachers equally for time worked in the morning and afternoons and take account of hours worked outside lesson time and outside school hours.
Martin Freedman, head of pay and conditions for the Association of Teachers and Lecturers, a RIG member, said: "My view is that the majority of part-time teachers will benefit from this." Some are expected to lose out under the deal which would be introduced from 2008 and cost up 0.23 per cent of the Pounds 19bn teacher pay bill.
But they would be entitled to three-year salary protection deals. RIG also wants to amend the regulations so that all teachers' pay decisions would be dependent on the outcomes of performance management reviews.
The NUT predicts this will lead to a "seismic" change for pay rises for teachers on the main scale.
But Chris Keates, NASUWT general secretary, said: "There is no intention to alter progression on the main scale." She said the RIG guidance made it clear that the current criteria which allowed automatic progression for all but the worst "unsatisfactory teachers" would continue.
Kerry George, NAHT head of pay and conditions, said the review body was being undermined by RIG.
It would have no choice but to change the pay regulations because a performance management system that did not fit the current document was already being consulted on.