what is ideal factor ratio

An ideal factor ratio is at which a producer or a firm will want to work.It is generally the second phase of production process when marginal product is falling but is positive and total product is is increasing at decreasing rate.This phase is called "ideal" because the first phase when MP is increasing,a firm has a scope of further increasing its TP,so it wishes to move to second phase.The third phase is undesirable as TP declines in this phase and MP is negative.So factor ratios prevailing in second phase is considered ideal for business. Moreover,a firm keeps on employing factor till the return from the factor (MP) is positive.The three phases are shown as I,II and III in following diagram :