Hi there! The above advice is great, and actually ties in nicely with an addon that I've been putting together that helps facilitate true solo play - Paragon Mode. Both systems essentially toughen up and increase the capabilities of a lone adventurer (or two) allowing you to throw more at them.
The playtest for FORGE Paragon Mode is currently on the Discord, if you wanted to check it out. It is functionally similar to Scarlet Heroes, however more tuned for the FORGE primary/secondary attributes and (hopefully) simplifies some things.
With regards to balancing encounters anyway, I suppose the normal response you will get is that OSR games are inherently unbalanced.. which isn't particularly helpful I know. I suppose you could say that a 1HD/1st Level enemy should be roughly equivalent to a 1st level Player Character, however that would also depend on the character's equipment and what capabilities the enemy had. I think it is important to remember that in OSR games it is generally not a good idea to try and go toe-to-toe with an enemy, as you would in more modern versions of D&D. Instead, you are encouraged to think outside the current combat engagement - can you use terrain/topple scenery/bypass the encounter with stealth or diplomacy? Ultimately it isn't as straightforward as you were hoping!
I think you get a feel for it, the more encounters you play/GM. I have seen a group of 6 level 1 adventurers clear out an entire goblin/bugbear cave (30+ enemies) with clever use of poison on their food supplies... there will almost always be a way for the players to win if they come up with a good plan!