Do not use any ratio, like 20%. See Návar (2015 FORest ECOlogy & Manage) the ratio varies with the age of the tree as well as with stand productivity or site index. A good approximatios to evaluate Below Ground Biomass (Root Biomass) is using the Cairns et al (1997) equation, as follows: EXP(-1.085+0.9256*LN(AGB)); Where: LN = Natural logarithm; AGB is the aboveground biomass content. AGB can be evaluated for tropical trees use the equations of Chave et al. (2013) for temperate or boreal trees use AGB = 0.2457pw*DBH^2.38; Where: pw is the wood basic specific gravity; DBH is diameter at breast height. For arid and semi arid trees use AGB = 0.0448 DBH^2.46 (See Navar et al., 2013 Journal of Arid Environments).

Good Luck with your evaluations

Kind Regards