The concentration percentage showed in commercial strong acids (e.g. HCl) is usually w/w, mass concentration.
Refill your agar with water as soon as you finish using it so that the residue can be easily cleaned.
The last digit on any digital-displayed equipment for measurement is not accurate.
The pipetters are most accurate just below the maximum volume. The more it deviates from that volume, the less accurate the measurement could be.
Do not autoclave or overheat agar with phosphate, and this is why.
Only use a high purity agar (e.g. Bacto agar) with minimal medium. If you are doing experiment using any kind of natural/semi natural medium (e.g. LB, BHI, AK, etc.), then use the much more expensive high purity agar makes no sense, because your natural medium contains much more unclear impurities in it.