view helpers generating assets paths with engine prefix for parent application assets

I want to create engines to embed in an application to add functionality but in most cases I want to use the parent application's layout and styles for a consistent look and feel. When accessing a view in an engine the assets included in the parent application's layout are prefixed with an auto generated engine prefix. I have an engine mounted at /svc with the following engine.rb file

module Svc
  class Engine < Rails::Engine
    config.asset_path = "/my_engine/%s"
  end
end

and the following helper reference in {app.root}/app/views/layouts/application.html.erb

    <%= stylesheet_link_tag "main" %>

When accessing /svc/index the output for this tag is

    <link href="/my_engine//stylesheets/main.css" media="screen" rel="stylesheet" type="text/css" />

removing the config.asset_path setting gives

 <link href="/svc_engine/stylesheets/main.css" media="screen" rel="stylesheet" type="text/css" />