First Project Completed: Workbench!

I took the advice of Ye Wreckers and built my first workbench (I had been leaning towards buying one). Since I don't know how to do fancy (or even non-fancy) joints, I bought some relatively straight construction-grade 2x4's (Douglas fir, I'm told yellow pine splits easily), some MDF and tempered hardboard, and knocked together a modest bench. It's fairly stable and has about 14 sq ft of work surface, and it only cost me about $50 in parts. Joints are all hardware-reinforced butt joints, so nothing too impressive there. But, I learned a ton. Mainly, that no matter what, your cuts and angles can never be too precise.
Anyway, I have a 46"x26" section of MDF and hardboard leftover, I was thinking of buying a few more 2x4's and making a low bench for additional storage and assembly. What height would you recommend for a generic, general-purpose secondary bench? My main bench is about 36" and it's just about perfect for me. I was thinking of 18-22" for the assembly table. It won't be anything fancy, it's mainly an opportunity to recycle some scrap and, as ever, hone my skills.
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