BackendPolicy size restrictions: Body size vs buffer size

We are currently in the process of migrating an nginx-ingress controller to Envoy Gateway, we have the following annotations on our nginx ingress resource:

```
nginx.ingress.kubernetes.io/proxy-body-size: 50m
nginx.ingress.kubernetes.io/proxy-buffer-size: 128k
nginx.ingress.kubernetes.io/proxy-busy-buffers-size: 256k
```

I can't figure out how to correctly rewrite this to Envoy Gateway syntax. I have created a BackendPolicy for my route with the following contents:

```
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: BackendTrafficPolicy
metadata:
name: my-site-policy
spec:
targetRefs:
- group: gateway.networking.k8s.io
kind: HTTPRoute
name: my-site-route
requestBuffer:
limit: 50m
```

However, I'm still unsure if this is the correct approach. I want the limit the request body to 50m, but that doesn't mean 50m should be allocated for every request.

Is this the correct approach, or are there better alternatives?