"agreed proportion" is correct and usable in written English.
You can use it to describe two or more parties coming to an agreement on how to divide a resource or responsibility. For example, "The board of directors and shareholders came to an agreed proportion of profits for the upcoming financial year."
Ukip and Liberal Democrat voters agreed in proportions of 63% and 58% respectively.
Approximately half of respondents thought that consent needed to be verbally agreed; a larger proportion of women than men incorrectly believed this was the law (AOR = 0.63, 95% CI = 0.44-0.90).
Outcomes of the recruitment process were measured as the proportion of practices that agreed to participate, the proportion of potentially eligible subjects who consented to take part in the trial and the attrition rate of subjects.
Irish fishermen have traditionally been paid by way of a share of the money made from the catch, divided in the proportions agreed by the owner.
Because Irish trawler crew have traditionally worked as share fishermen – a system whereby workers are paid a share of the money made from the catch, divided in proportions agreed by the owner – they have generally been considered self-employed and responsible for their own tax and insurance.
Similar proportions agreed that recommended adult vaccines should be publicly funded (81.8 91.6% of HCPs and 78.1% of the public).
Smaller proportions agreed that the standard of care was being met in the province as a whole, or that Ontario compared favourably to other jurisdictions in Canada or the US in ensuring access.
The majority of senior chiropractic, naturopathic, physical and occupational therapy students who responded to our survey agreed in similar proportions that fibromyalgia was difficult to manage, but the majority expressed confidence that effective treatment options existed (57% in our sample), and in particular naturopathic students (79%).
