Strait pro-mix HP for all stages...you'll never go back. I've done comparison side by sides with other formulations and blends. Best preformance with greatest ease of use.
I ve used bx for a number of years.....I dont add extra perlite.....
Never tried hp.....
and generally I dont water bx enough for much runoff......
Since hp is high porosity...... do you water enough to get significant run off????