A key feature of most past the post is the threshold
The threshold is set in advance and determines how the results are calculated.
It defines what share of the overall vote a party must receive in order to qualify for proportionally allocated seats.
In an election with a threshold set at 3% the party getting 3.2% of the vote will get their share of the national vote translated in number of seats.
The party getting 2.25% of the vote will only win seats in constituencies where their candidate got the most votes.
Parties polling above the threshold qualify for proportionally calculated seats under most past the post.
Parties polling below the threshold can still win seats but only if they win them outright under first past the post rules.
Where to set the threshold value would be set by a mechanism defined in electoral legislation.
For more information on electoral thresholds see the wikipedia article on the subject.