Pro-Rata Negotiations

A large percentage of pre-seed and seed investments are made with YC SAFE notes, in those cases the YC pro rata side letter is the accepted baseline, which provides a right to purchase shares in the following equity (conversion) round at the ratio of the SAFE investment to company capitalization. The YC templates are at https://www.ycombinator.com/documents
For priced rounds (stock purchase) in the US, NVCA model documents are an accepted baseline https://nvca.org/model-legal-documents/
Regardless of investment vehicle, it is important for VCs to be fair in negotiation and especially not place demands on founders or create terms that could prove toxic to future fundraising efforts.