Fuzzy cactus

Playing with 4x5 macro, I tried out a potted cactus under the White Lightning unit. First try at metering with the Minolta ambient flash meter resulted in an error message, the flash was too darn bright! So I grabbed my spotmeter and used a grey card and finally got a reading of f/90, which by my calculations led to setting the lens at 22.5. Using the 65mm had the camera about three inches from the cactus which made it a bit interesting focusing using about 200mm of bellows draw and about 1.5 times life size image. Image still looked a bit soft under the loupe when I examined the neg on the light table. I'm wondering what else I could do to get a sharper image on the film using this combo. Or perhaps my 150 will do better for image size with more bellows draw? Curious to know others experience with a similar setup.