Provisioning SQL Server 2019
If you only have one drive, you won't gain performance by splitting it logically but you said you want to split the drive into logical drives for logs and data so if one runs out of room it will not cripple the others' content. However, if either your data drive or your log drive or your tempdb drive is out of space then you are down. I would instead keep on the same drive in order to maximize the space you do have. It really sounds like you have plenty of room for two 35 GB databases. Monitor your mdfs and ldfs for auto growths and your drive for free space. Move your system databases to the same drive. Compress your databases. If the databases are not in simple recovery model, be sure to take frequent log backups. Compress your backups. Make sure you are copying your backups to another location so you have a copy in case your drive crashes. Make sure you have jobs to prune msdb history and old backups.
I wouldn't size my log files or my tembdb based on a rule of 25% or anything else. Size them pretty small to start and watch them autogrow and then resize them to that.