Darparodd Comisiwn Holtham ddadansoddiad trylwyr o sut y gallai'r grant bloc gael ei leihau i gyfrif am y refeniw o drethi datganoledig yng nghymru. Ategodd Comisiwn Silk y dadansoddiad hwnnw gan ddod i'r casgliad “y byddai’n amhriodol i Lywodraeth Cymru fod yn agored i’r holl risgiau sy’n gysylltiedig â datganoli pwerau trethu gan ei bod yn bosibl na fydd rhai o’r risgiau hynny’n ganlyniad i weithredoedd gan Lywodraeth Cymru. Er hynny, mae’n briodol bod cyllid Cymru’n agored i’r risgiau sy’n gysylltiedig â dewisiadau gan Lywodraeth Cymru”.
The Holtham Commission provided a thorough analysis of how the block grant could be reduced to account for the revenues from devolved taxes in Wales.  The Silk Commission endorsed that analysis and concluded that: "it would be inappropriate for the Welsh Government to be exposed to all the risks associated with the devolution of tax powers as some of this risk may not be the result of actions by the Welsh Government.  It is, however, right that the Welsh budget should be exposed to the risks associated with the choices of the Welsh Government".