While I love this advice in general it will not help @JonP, since the method applied will only work on links with ATM cell quantization. It will not work on cable/DOCISIS links nor on PTM using VDSL2 links.
This however is good advice, except I will always recommend to leave the "symbolic" overhead keywords alone and use "overhead 18 mpu 64" instead, assuming your cake and tc versions support the mpu keyword. MPU makes sure that the shaper will account for the fact that L2 ethernet frames including the FCS will always be >= 64Byte in length.
To cite the relevant section from the Docsis standard (http://www.cablelabs.com/specification/docsis-3-0-mac-and-upper-layer-protocols-interface-specification/):
"C.2.2.7.2 Maximum Sustained Traffic Rate 632 This parameter is the rate parameter R of a token-bucket-based rate limit for packets. R is expressed in bits per second, and MUST take into account all MAC frame data PDU of the Service Flow from the byte following the MAC header HCS to the end of the CRC, including every PDU in the case of a Concatenated MAC Frame. This parameter is applied after Payload Header Suppression; it does not include the bytes suppressed for PHS. The number of bytes forwarded (in bytes) is limited during any time interval T by Max(T), as described in the expression: Max(T) = T * (R / 8) + B, (1) where the parameter B (in bytes) is the Maximum Traffic Burst Configuration Setting (refer to Annex C.2.2.7.3). NOTE: This parameter does not limit the instantaneous rate of the Service Flow. The specific algorithm for enforcing this parameter is not mandated here. Any implementation which satisfies the above equation is conformant. In particular, the granularity of enforcement and the minimum implemented value of this parameter are vendor specific. The CMTS SHOULD support a granularity of at most 100 kbps. The CM SHOULD support a granularity of at most 100 kbps. NOTE: If this parameter is omitted or set to zero, then there is no explicitly-enforced traffic rate maximum. This field specifies only a bound, not a guarantee that this rate is available."
So in essence DOCSIS users need to only account for 18 Bytes of ethernet overhead in both ingress and egress directions under non-congested conditions. And under congestion this does not matter anymore for sqm, since then the problem is that the configured shaper rates will be too high anyway...