Overall, the Remain United calculations are that tactical voting for the best placed Remain party in each Euro-constituency would boost the number of Remain MEPs by more than half. That gives a powerful incentive to Remainers to cast their votes tactically.
The reason that tactical voting is effective even in the European Parliament elections despite their use of PR is due to the version of PR being used. The d’Hondt quota with closed candidate lists means there is plenty of scope for tactical voting to have an impact.
Details of the Remain United research and recommendations are here.